U.S. patent number 7,224,519 [Application Number 10/798,662] was granted by the patent office on 2007-05-29 for low noise multi-wavelength light source and wavelength division multiplexing system using same.
This patent grant is currently assigned to Samsung Electronics Co., Ltd.. Invention is credited to Seong-Taek Hwang, Dae-Kwang Jung, Jeong-Seok Lee, Hyun-Cheol Shin.
United States Patent |
7,224,519 |
Shin , et al. |
May 29, 2007 |
Low noise multi-wavelength light source and wavelength division
multiplexing system using same
Abstract
A multi-wavelength light source includes a substrate, a
fabry-perot laser laminated on the substrate that is operated by
driving current below a predetermined threshold current to generate
multi-wavelength light including a plurality of peaks whose
wavelengths and spacing are identical to these of WDM channels. A
semiconductor optical amplifier (SOA) is laminated on the substrate
in an arrangement such that a slant surface of the SOA is opposed
to a side surface of the fabry-perot laser, which serves to thereby
amplify the multi-wavelength light output from the fabry-perot
laser. The semiconductor optical amplifier is driven in a gain
saturation state to reduce the relative intensity of noise in the
channels of the multi-wavelength light that are simultaneously
amplified.
Inventors: |
Shin; Hyun-Cheol (Suwon-si,
KR), Lee; Jeong-Seok (Anyang-si, KR),
Hwang; Seong-Taek (Pyeongtaek-si, KR), Jung;
Dae-Kwang (Suwon-si, KR) |
Assignee: |
Samsung Electronics Co., Ltd.
(Suwon-Si, Gyeonggi-Do, KR)
|
Family
ID: |
34214744 |
Appl.
No.: |
10/798,662 |
Filed: |
March 11, 2004 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20050047727 A1 |
Mar 3, 2005 |
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Foreign Application Priority Data
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Aug 29, 2003 [KR] |
|
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10-2003-0060191 |
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Current U.S.
Class: |
359/344;
372/50.22; 398/71 |
Current CPC
Class: |
H01S
5/0265 (20130101); H01S 5/14 (20130101); H01S
5/028 (20130101); H01S 5/1085 (20130101); H01S
5/1092 (20130101); H01S 5/06812 (20130101) |
Current International
Class: |
H01S
5/50 (20060101); H04J 14/02 (20060101) |
Field of
Search: |
;398/72,183,71
;372/50.22,50.121 ;359/344 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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62-199086 |
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Sep 1987 |
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JP |
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03-163891 |
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Jul 1991 |
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JP |
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05-206570 |
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Aug 1993 |
|
JP |
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2002-270949 |
|
Sep 2002 |
|
JP |
|
2003-134058 |
|
May 2003 |
|
JP |
|
WO 03/063401 |
|
Jul 2003 |
|
WO |
|
Other References
Lasing threshold. Wikipedia, the free encyclopedia. Multiple
Editors/Authors. Uploaded: Dec. 6, 2005. Downloaded: Jan. 11, 2006.
http://en.wikipedia.org/w/index.php?title=Lasing.sub.--threshold&oldid=30-
321671. cited by examiner .
Spectral linewidth. Wikipedia, the free encyclopedia. Multiple
Editors/Authors. Uploaded: Nov. 2, 2005. Downloaded: Jan. 11, 2006.
http://en.wikipedia.org/w/index.php?title=Spectral.sub.--linewidth&oldid=-
27157822. cited by examiner.
|
Primary Examiner: Keith; Jack
Assistant Examiner: Diacou; Ari M
Attorney, Agent or Firm: Cha & Reiter, LLC
Claims
What is claimed is:
1. A multi-wavelength light source comprising: a substrate; a laser
laminated on a first portion of the substrate wherein said laser
configured to generate multi-wavelength light including a plurality
of channels having different wavelengths when driven by a driving
current below a predetermined threshold current; and a
semiconductor optical amplifier being laminated on a second portion
of the substrate and having a first end surface, the first end
surface being slanted and opposed to a side surface of the laser,
wherein the semiconductor optical amplifier is configured to reduce
a relative intensity noise in the plurality of channels of the
multi-wavelength light and to amplify the multi-wavelength light
output from the laser simultaneously, and wherein a band gap of the
semiconductor optical amplifier is smaller than that of the
laser.
2. The multi-wavelength light source as claimed in claim 1, wherein
the laser comprises a fabry-perot laser, and the multi-wavelength
light source further comprises: a high reflection layer coated on a
first end surface of the multi-wavelength light source, the first
end surface of the multi-wavelength light source including a first
end surface of the fabry-perot laser; and anti-reflection layers
being arranged on a side surface of the fabry-perot laser, the
slanted surface of the semiconductor optical amplifier, and a
second end surface of the multi-wavelength light source, wherein
the second end surface of the multi-wavelength light source
includes a second end surface of the semiconductor optical
amplifier means.
3. The multi-wavelength light source as claimed in claim 1, wherein
a spectrum of the multi-wavelength light outputted from the laser
coincides with a gain spectrum that is amplified by the
semiconductor optical amplifier.
4. The multi-wavelength light source as claimed in claim 1, wherein
the slanted surface of the semiconductor optical amplifier opposed
to the side surface of the laser is inclined at a predetermined
angle with respect to the side surface of the laser.
5. A multi-wavelength light source comprising: a fabry-perot laser
configured to generate multi-wavelength light including a plurality
of peaks having different wavelengths when driven by driving a
current below a predetermined threshold current; and a
semiconductor optical amplifier being coupled to an output of the
fabry-perot laser and being configured to amplify the
multi-wavelength light outputted from the fabry-perot laser,
wherein the semiconductor optical amplifier means is configured to
reduce a relative intensity of noise in the plurality of channels
of the multi-wavelength light and amplify the multi-wavelength
light simultaneously, and wherein a band gap of the semiconductor
optical amplifier is smaller than that of the fabry-perot
laser.
6. The multi-wavelength light source as claimed in claim 5, wherein
a spectrum of the multi-wavelength light output from the
fabry-perot laser coincides with a gain that is amplified by the
semiconductor optical amplifier.
7. A wavelength division multiplexing system comprising a central
office, a remote node coupled to the central office by an optical
fiber, and a plurality of subscribers connected to the remote node,
the central office comprising: a light source section including a
laser and a semiconductor optical amplifier, the laser configured
to be driven by driving current below threshold current and
configured to generate multi-wavelength light including a plurality
of downstream channels having different wavelengths, and the
semiconductor optical amplifier configured to amplify the
multi-wavelength light in a gain saturation state and to output the
amplified multi-wavelength light; a demultiplexer configured to
demultiplex the multi-wavelength light into a plurality of
downstream channels having different wavelengths and to output the
demultiplexed downstream channels; a first
multiplexer/demultiplexer configured to demultiplex an upstream
optical signal outputted from the remote node into a plurality of
upstream channels having different wavelengths and configured to
multiplex the downstream channels into a downstream optical signal
so as to output the multiplexed optical signal to the remote node;
and a plurality of photodetectors configured to detect the upstream
channels demultiplexed by the first multiplexer/demultiplexer,
wherein a band gap of the semiconductor optical amplifier is
smaller than that of the laser.
8. The multi-wavelength light source as claimed in claim 7, wherein
the laser of the light source section includes a fabry-perot
laser.
9. The multi-wavelength light source as claimed in claim 7, wherein
the central office further comprises: a plurality of modulators
configured to modulate the downstream channels demodulated by the
demultiplexer; and a plurality of wavelength selection couplers
located between each of the modulators and the first
multiplexer/demultiplexer and configured to output the downstream
channels that are output from the modulators to the first
multiplexer/demultiplexer and to output the upstream channels,
which are outputted from the first multiplexer/demultiplexer, to a
corresponding photodetector.
10. The multi-wavelength light source as claimed in claim 7,
wherein the remote node includes a second multiplexer/demultiplexer
configured to multiplex a plurality of upstream channels having
different wavelengths, which are output from the subscribers, into
an upstream optical signal, to output the multiplexed optical
signal to the central office, to demultiplex the downstream optical
signal output from the central office into a plurality of
downstream channels, and to output the demultiplexed downstream
channels to a corresponding subscriber.
11. The multi-wavelength light source as claimed in claim 7,
wherein each subscriber comprises: a photodetector configured to
detect a corresponding downstream channel; a light source
configured to output the upstream channel to the remote node; and a
wavelength selection coupler configured to output the downstream
channel to the photodetector and to output the upstream channel
generated by the light source to the remote node.
Description
CLAIM OF PRIORITY
This application claims priority to an application entitled
"Multi-wavelength light source and wavelength division multiplexing
system using the same," filed in the Korean Intellectual Property
Office on Aug. 29, 2003 and assigned Serial No. 2003-60191, the
contents of which are hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to light sources that are used for
optical communications. More particularly, the present invention
relates to a multi-wavelength light source which can output
multi-wavelength light in a multiplex system that includes a
plurality of channels having differing wavelengths.
2. Description of the Related Art
In wavelength division multiplexed optical communication systems, a
plurality of channels having different wavelengths are multiplexed
into as a composite optical signal that is transmitted across a
transmission medium. The multiplexed optical signal is received and
demultiplexed into a plurality of signals having different
wavelengths, and each of these demultiplexed signals are detected
and grouped as separate channels according to their wavelengths.
Accordingly, wavelength division multiplexing methods permit the
efficient expansion of optical communication capacity, and allows
for data to be transmitted regardless of the transmission data
type.
A typical wavelength division multiplexed optical communication
system includes a central office and at least one remote node. In
order to transmit data to subscribers, the central office
multiplexes a plurality of downstream channels having different
wavelengths into a downstream optical signal so as to output the
multiplexed optical signal. Further, the central office detects
upstream channels output from the subscribers. The remote node is
located between the subscribers and the central office to relay the
central office to each subscriber.
In the wavelength division multiplexing, some of the types of light
sources that can be used are a plurality of single-wavelength light
sources or an incoherent multi-wavelength light source. Distributed
feedback lasers or fabry-perot lasers can be used as the
single-wavelength light sources, and an erbium doped fiber
amplifier or a light emitting diode (LED) can be used as the
incoherent multi-wavelength light source.
Each of the single-wavelength light sources generates only one
mode-locked channel so as to have a single wavelength by a laser
resonance. Therefore, the single-wavelength light sources are
advantageous in long-distance transmissions because they are
coherent sources. Also, the channel power loss and occurrences of
noise are minimized in the single-wavelength light sources.
However, one drawback of using single-wavelength light sources is
that a plurality of single-wavelength light sources must be
provided to the system in order to correspond respectively to the
number of transmitted channels. This one to one correspondence
increases not only the size of the wavelength division multiplexed
optical communication system increases, but also the manufacturing
costs.
Meanwhile, the multi-wavelength light sources, such as the LED,
have been proposed as an alternative for solving the
above-mentioned problems of the single-wavelength light sources.
However, the multi-wavelength light sources have the problem that
they output incoherent light. Therefore, in comparison with the
single-wavelength light source, the multi-wavelength light sources
are disadvantageous in a long-distance transmission.
In order to overcome the disadvantages of both the multi-wavelength
light source and the single-wavelength light source, a fabry-perot
laser with EDFA (Erbium Doped Fiber Amplifier) has been proposed
for use in a method of generating and amplifying multi-wavelength
light including channels having different wavelengths.
However, the fabry-perot laser does not solve all the
above-mentioned problems, particularly when used in
multi-wavelength light source. The multiple channels output from
the laser are subject to power fluctuation that commonly occurs,
thereby increasing relative intensity noise.
SUMMARY OF THE INVENTION
Accordingly, the present invention provides a multi-wavelength
light source that has both a low relative intensity of noise and a
low manufacturing cost.
According to a first aspect of the present invention, there is
provided a multi-wavelength light source comprising: a substrate, a
fabry-perot laser laminated on the substrate, and driven by driving
current below threshold current, thereby generating
multi-wavelength light including a plurality of peaks whose
wavelengths and spacing are identical to these of WDM channels, and
a semiconductor optical amplifier laminated on the substrate in
such a manner that a slant surface of the semiconductor optical
amplifier is opposed to a side surface of the fabry-perot laser,
thereby amplifying the multi-wavelength light outputted from the
fabry-perot laser, wherein the semiconductor optical amplifier is
operated in a gain saturation state, thereby reducing the relative
intensity noise of channels of the multi-wavelength light and
simultaneously amplifying the multi-wavelength light.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and other objects, features and advantages of the present
invention will be more apparent from the following detailed
description taken in conjunction with the accompanying drawings, in
which:
FIG. 1 is a perspective view of a multi-wavelength light source
according to a first aspect of the present invention, which
includes a fabry-perot laser and a semiconductor optical amplifier
integrated on a single-substrate;
FIG. 2 is a plan view of the multi-wavelength light source shown in
FIG. 1;
FIG. 3 is a graph of a gain curve showing variation of power of
multi-wavelength lights input to and amplified by the semiconductor
optical amplifier shown in FIG. 1;
FIG. 4 is a graph showing the power of the multi-wavelength lights
shown in FIG. 3 input to the semiconductor optical amplifier;
FIG. 5 is a graph showing the power of the multi-wavelength lights
shown in FIG. 3 amplified by the semiconductor optical
amplifier;
FIG. 6 is a block diagram showing a construction of a wavelength
division multiplexing system according to a second aspect of the
present invention; and
FIG. 7 is a block diagram showing a construction of a wavelength
division multiplexing system according to a third aspect of the
present invention.
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, a description of the present invention will be
presented with reference to the accompanying drawings. For the
purposes of clarity and simplicity, a detailed description of known
functions and configuration incorporated herein will be omitted as
it may obscure the subject matter of the present invention.
FIG. 1 is a perspective view of a multi-wavelength light source
according to a first aspect of the present invention, which
includes a fabry-perot laser and a semiconductor optical amplifier
integrated on a single-substrate. Referring to FIG. 1, the
multi-wavelength light source 100 according to the first aspect of
the present invention includes a substrate 110, a fabry-perot laser
120 driven by driving current below threshold current, and the
semiconductor optical amplifier (hereinafter, referred to as a SOA)
130. The multi-wavelength light source 100 has a high reflection
layer 101 coated on a first surface of the multi-wavelength light
source 100 including an end surface of the fabry-perot laser 120.
Further, the multi-wavelength light source 100 has anti-reflection
layers, which are coated on a side surface 120a of the fabry-perot
laser 120 opposed to the SOA 130, a slant surface 130a of the SOA
130 opposed to the fabry-perot laser 120, and a second end surface
102 of the multi-wavelength light source 100 including one surface
of the SOA 130, respectively.
The fabry-perot laser 120 is laminated on the substrate 110 and is
operated by driving current below a predetermined threshold
current, thereby causing the output of multi-wavelength light
including a plurality of peaks whose wavelengths and spacing are
identical to these of WDM channels, The SOA 130 is laminated on the
substrate 110 in such a manner that end surfaces 120a and 130a of
the fabry-perot laser 120 and the SOA 130 are opposed to each
other. The arrangement of the opposing ends of the SOA and the
fabry-perot laser permit the amplifying of the multi-wavelength
light output from the fabry-perot laser 120.
FIG. 2 is a plan view of the multi-wavelength light source shown in
FIG. 1. Referring to FIG. 2, the end surface 130a of the SOA 130
opposed to end surface 120a of the fabry-perot laser 120 is
inclined with respect to a predetermined angle, with thereby
preventing the multi-wavelength light output from the fabry-perot
laser 120 from being reflected from the SOA 130 to the fabry-perot
laser 120. Further, a band gap of the SOA 130 is smaller than that
of the fabry-perot laser 120, so that a spectrum of the
multi-wavelength light output from the fabry-perot laser 120
coincides with a gain spectrum that can be amplified by the SOA
130. The anti-reflection layers are coated on the slant surface
130a of the SOA 130, which is opposed to the fabry-perot laser 120,
and the side surface 120a of the fabry-perot laser 120 opposed to
the SOA 130.
The peak wavelengths of output channels as well as the intervals
between the outputted channels are determined according to the
length of the fabry-perot laser 120. Accordingly, when the
multi-wavelength light source shown in FIG. 2 is employed in the
wavelength division multiplexing system, the length of the
fabry-perot laser 120 is adjusted, so that wavelengths of channels
and intervals between the channels can be adjusted, which are
necessary for the wavelength division multiplexing system.
FIG. 3 is a graph of a gain curve showing correlation between the
power of multi-wavelength lights input to the SOA shown in FIG. 1,
and power of the multi-wavelength lights amplified by the SOA.
Further, FIG. 4 shows the power of the multi-wavelength lights
inputted to the SOA shown in FIG. 1, and FIG. 5 shows the power of
the multi-wavelength lights amplified by the SOA shown in FIG. 1.
Hereinafter, an operation characteristic of the multi-wavelength
light source shown in FIG. 1 according to the first embodiment of
the present invention will be described with reference to FIGS. 3
to 5.
The gain curve of the SOA shown in FIG. 3 shows variation of the
power of the multi-wavelength lights 310, 320 input to and
amplified by the SOA versus the output values 311, 321,
respectively. The graph has a linear region and a gain saturation
region. In the linear region, as the power of the multi-wavelength
lights input to the SOA increases, the power of the amplified
multi-wavelength light output gradually increases. In the gain
saturation region, the output power doesn't increase even if the
input power is increased gradually. FIG. 4 is a graph showing the
power of the multi-wavelength lights 310, 320 input to the SOA 130
shown in FIG. 1. The multi-wavelength lights are output from the
fabry-perot laser 120. First multi-wavelength light 310 (which is
shown in FIG. 3) has power that corresponds to the gain saturation
region of the SOA, which is larger than the power of second
multi-wavelength light 320.
Referring back to FIG. 3, the second multi-wavelength light 320 is
amplified in the linear region of the SOA instead of the gain
saturation region because the second multi-wavelength light has
less power than the first multi-wavelength light 310. The first and
the second multi-wavelength lights 310 and 320 include a plurality
of channels having different wavelengths, and the power of each
channel (as shown by pins 1 and 2 in FIG. 4) varies over time. This
characteristic is referred to as power fluctuation, and is
generally not desirable.
FIG. 5 is a graph showing the output power of the multi-wavelength
lights after being amplified by the SOA 130 shown in FIG. 1. Thus,
according to FIG. 5, pins 1 and 2 represent the respective output
power of multi-wavelength lights 310 and 320 after amplification by
the SOA. It can be seen that pin 1 output power, although being
larger than that of pin 2, has a lower amount of power fluctuation.
As described above, in the linear region of the SOA, the power of
the amplified multi-wavelength lights increases in proportion to
the power of the input multi-wavelength lights. Therefore, it can
be shown from FIG. 5 the different in power fluctuation of the
second multi-wavelength light 320 inputted to the SOA versus the
first multi-wavelength light 310. Due to the fact that the first
multi-wavelength light 310 is input to the gain saturation region
(shown in FIG. 3), wherein the output power is almost constant as
the input power is varying, the power fluctuation of the multiple
channels included in first multi-wavelength light is also
reduced.
That is, the power of the multi-wavelength light outputted from the
fabry-perot laser 120 shown in FIG. 1 has power included in the
gain saturation region of the SOA, so that the power fluctuation of
the multi-wavelength light is also reduced. Furthermore, the
reduction of the power fluctuation decreases relative intensity
noise.
FIG. 6 is a block diagram showing a wavelength division multiplexed
communication system including a multi-wavelength light source
according to a second embodiment of the present invention. The
wavelength division multiplexed communication system includes a
central office 200 outputting multi-wavelength light, a remote node
230 connected to the central office 200 through an optical fiber,
and a plurality of subscribers 240-1 to 240-n connected to the
remote node 230.
The central office 200 includes a light source section 210 for
generating multi-wavelength light comprising the fabry-perot laser
211 and the SOA 212, wherein the output of the light source section
211 is demultiplexed by, a demultiplexer 220, a plurality of
modulators 201-1 to 201-n for modulating the respective multiplexed
signals individually, a plurality of photodetectors 203-1 to 203-n
for detecting upstream channels sent by subscribers via the remote
node 230 and demultiplexed by a first Mux/Demux 221 and a plurality
of wavelength selection couplers 202-1 to 202-n.
The light source section 210 includes a laser 211 and a
semiconductor optical amplifier (hereinafter, referred to as a SOA)
212. The laser 211 is operated by driving current below a
predetermined threshold current so as to generate the
multi-wavelength light including a plurality of downstream channels
having different wavelengths. The SOA 212 amplifies the
multi-wavelength light in a gain saturation state so as to output
the amplified light. Therefore, the light source section 210
amplifies the multi-wavelength light while reducing relative
intensity noise of the downstream channels of the multi-wavelength
light. A fabry-perot laser, etc., can be used as the laser 211, and
while preferred, is not required.
The demultiplexer 220 demultiplexes the multi-wavelength light
generated by the light source section 210 into a plurality of
downstream channels 205 having different wavelengths, and outputs
the demultiplexed downstream channels to the modulators 201-1 to
201-n. An arrayed waveguide grating, etc., can be used as the
demultiplexer 220.
The first multiplexer/demultiplexer 221 demultiplexes an upstream
optical signal outputted from the remote node 230 into a plurality
of upstream channels having different wavelengths, and outputs the
demultiplexed upstream channels to the photodetectors 203-1 to
203-n. Further, the first multiplexer/demultiplexer 221 multiplexes
the downstream channels modulated by the modulators
201-1.about.201-n into a downstream optical signal, and outputs the
multiplexed optical signal to the remote node 230.
The photodetectors 203-1 to 203-n detect the upstream channels that
are demultiplexed by the first multiplexer/demultiplexer 221, and
they include light-receiving type active elements such as photo
diodes, etc.
The wavelength selection couplers 202-1 to 202-n outputs the
downstream channels modulated by the modulators 201-1 to 201-n to
the first multiplexer/demultiplexer 221, and outputs the upstream
channels, which are output from the first multiplexer/demultiplexer
221, to a corresponding photodetector 203-1 or 203-n.
The remote node 230 includes a second multiplexer/demultiplexer
231, so that the remote node 230 multiplexes a plurality of
upstream channels having different wavelengths, which are output
from the subscribers 240-1 to 240-n, into an upstream optical
signal, and outputs the multiplexed optical signal to the central
office 200. Further, the remote node 230 demultiplexes the
downstream optical signal output from the central office 200 into a
plurality of downstream channels, and outputs the demultiplexed
downstream channels to the subscribers 240-1 to 240-n.
Each of the subscribers 240-1 to 240-n includes a photodetector
242, a light source 243, and a wavelength selection coupler
241.
The wavelength selection coupler 241 outputs the downstream
channel, which is output from the remote node 230, to the
photodetector 242, and outputs an upstream channel generated by the
light source 243 to the remote node 230.
The photodetector 242 detects a corresponding downstream channel
outputted from the remote node 230, and includes a photo diode,
etc.
The light source 243 outputs the upstream channel to the wavelength
selection coupler 241, and includes a semiconductor laser, etc.
FIG. 7 is a block diagram showing a construction of a wavelength
division multiplexing system according to a third aspect of the
present invention. Referring to FIG. 7, the wavelength division
multiplexing system includes a central office 300 generating a
downstream optical signal, a remote node 340 demultiplexing the
downstream optical signal into a plurality of downstream channels
having different wavelengths, and a plurality of subscribers 350-1
to 350-n connected to the remote node 340.
The central office 300 includes a plurality of light sources 313-1
to 313-n for generating mode-locked downstream channels, a
plurality of photodetectors 311-1 to 311-n for detecting upstream
channels, a first multiplexer/demultiplexer 312, a downstream
broadband light source 330, an upstream broadband light source 320,
an optical coupler 310, and wavelength selection couplers 314-1 to
314-n.
Each of the light sources 313-1 to 313-n generates mode-locked
downstream channels having different wavelengths by corresponding
incoherent light.
Each of the photodetectors 311-1 to 311-n detects a corresponding
upstream channel outputted from the first multiplexer/demultiplexer
312.
Each of the wavelength selection couplers 314-1 to 314-n outputs
incoherent lights, which are outputted from the first
multiplexer/demultiplexer 312, to the light sources 313-1 to 313-n,
and outputs the upstream channels, which are outputted from the
first multiplexer/demultiplexer 312, to the photodetectors 311-1 to
311-n. Further, each of the wavelength selection couplers 314-1 to
314-n outputs the downstream channels generated by the light
sources 313-1 to 313-n to the first multiplexer/demultiplexer
312.
The downstream broadband light source 330 includes a first laser
333 for generating downstream light, a first semiconductor optical
amplifier 332, and a first isolator 331, so that the downstream
broadband light source 330 enables the light sources 313-1 to 313-n
to generate the mode-locked downstream channels.
The first laser 333 can use a fabry-perot laser and is driven by
driving current below threshold current, so that the first laser
333 generates downstream light, which includes a plurality of
incoherent lights having different wavelengths, for mode-locking
the light sources 313-1 to 313-n.
The first semiconductor optical amplifier 332 amplifies the
downstream light, which is generated by the first laser 333, in a
gain saturation state, thereby reducing power fluctuation of the
downstream light and relative intensity noise of the downstream
light due to the power fluctuation.
The first isolator 331 is connected to the optical coupler 310, so
that the first isolator 331 outputs the downstream light, which is
amplified by the first semiconductor optical amplifier 332, to the
optical coupler 310, and reflects an upstream optical signal, which
is outputted from the optical coupler 310, to the optical coupler
310.
The upstream broadband light source 320 includes a second laser 323
for generating upstream light, a second semiconductor optical
amplifier 322, and a second isolator 321, and outputs upstream
light, which includes a plurality of incoherent lights, for
mode-locking the subscribers 350-1 to 350-n.
The second laser 323 can use a fabry-perot laser and is driven by
driving current below threshold current, so that the second laser
323 generates upstream light, which includes a plurality of
incoherent lights having different wavelengths, for mode-locking
the subscribers 350-1 to 350-n.
The first semiconductor optical amplifier 322 amplifies the
upstream light, which is generated by the second laser 323, in a
gain saturation state, thereby reducing power fluctuation of the
upstream light and relative intensity noise of the upstream light
due to the power fluctuation.
The second isolator 321 is connected to the optical coupler 310, so
that it outputs the upstream light, which is amplified by the
second semiconductor optical amplifier 322, to the optical coupler
310, and reflects a downstream optical signal, which is outputted
from the optical coupler 310, to the optical coupler 310.
The first multiplexer/demultiplexer 312 demultiplexes the
downstream light generated by the downstream broadband light source
330 into a plurality of incoherent lights so as to output the
demultiplexed incoherent lights to the wavelength selection
couplers 314-1 to 314-n. Further, the first
multiplexer/demultiplexer 312 multiplexes the downstream channels
generated by the light sources 313-1 to 313-n into a downstream
optical signal so as to output the multiplexed optical signal to
the optical coupler 310. Furthermore, the first
multiplexer/demultiplexer 312 demultiplexes the upstream optical
signal outputted from the optical coupler 310 into upstream
channels having different wavelengths so as to output the
demultiplexed upstream channels to the photodetectors 311-1 to
311-n.
The optical coupler 310 outputs the downstream light and the
upstream optical signal to the first multiplexer/demultiplexer 312,
and outputs the upstream light and the downstream optical signal to
the remote node 340.
The remote node 340 includes a second multiplexer/demultiplexer
341. The multiplexer/demultiplexer 341 demultiplexes the downstream
optical signal outputted from the optical coupler 310 into
downstream channels having different wavelengths so as to output
the demultiplexed downstream channels to the subscribers 350-1 to
350-n. Further, the second multiplexer/demultiplexer 341
multiplexes a plurality of upstream channels having different
wavelengths generated by the subscribers 350-1 to 350-n into an
upstream optical signal so as to output the multiplexed optical
signal to the central office 300. Furthermore, the second
multiplexer/demultiplexer 341 demultiplexes the upstream light
outputted from the central office 300 into a plurality of
incoherent lights having different wavelengths so as to output the
demultiplexed incoherent lights to the subscribers 350-1 to
350-n.
Each of the subscribers 350-1 to 350-n includes a light source 353,
and a photodetector 352, and a wavelength selection coupler 351.
The light source 353 generates a mode-locked upstream channel by
corresponding incoherent light, and the photodetector 352 detects a
corresponding downstream channel. The wavelength selection coupler
351 outputs the downstream channel, which is outputted from the
remote node 340, to the photodetector 352, and outputs the
incoherent light to the light source 353. Further, the wavelength
selection coupler 351 outputs the mode-locked upstream channel
generated by the light source 353 to the remote node 340.
The light source 353 includes a fabry-perot laser, etc., and the
photodetector 352 includes a photo diode, etc.
That is, according to the present invention, a fabry-perot laser is
driven by driving current below threshold current so as to generate
multi-wavelength light including a plurality of channels having
different wavelengths. Further, the generated multi-wavelength
light is amplified by a semiconductor optical amplifier, which has
been driven by high driving current and in a gain saturation state,
so as to generate a multi-wavelength light in which a gain of each
channel is maintained constant.
According to a multi-wavelength light source of the present
invention, multi-wavelength light having a plurality of channels
different from each other, which is generated in a fabry-perot
laser before a laser resonance, is amplified by a semiconductor
optical amplifier in a gain saturation state, so that power
fluctuation of each channel is reduced, and thus relative intensity
noise is also reduced. Further, a semiconductor optical amplifier
and a fabry-perot laser are easily integrated, so that a
multi-wavelength light source having a miniaturized size can be
manufactured, and the manufacturing cost is reduced.
While the invention has been shown and described with reference to
certain preferred embodiments thereof, it will be understood by
those skilled in the art that various changes in form and details
may be made therein without departing from the spirit and scope of
the invention as defined by the appended claims.
* * * * *
References